[LLVMdev] Is LLVM expressive enough to represent asynchronous exceptions?

[John McCall]

On Jun 12, 2011, at 11:24 PM, Bill Wendling wrote:

> On Jun 12, 2011, at 4:40 PM, John McCall wrote:
> 
>> On Jun 12, 2011, at 2:14 PM, Cameron Zwarich wrote:
>> 
>>> On Jun 12, 2011, at 1:25 AM, Duncan Sands wrote:
>>> 
>>>> Hi Sohail,
>>>> 
>>>>> Is LLVM expressive enough to represent asynchronous exceptions?
>>>> 
>>>> not currently.  The first step in this direction is to get rid of the invoke
>>>> instruction and attach exception handling information to basic blocks.  See
>>>> http://nondot.org/sabre/LLVMNotes/ExceptionHandlingChanges.txt
>>>> for a discussion.
>>> 
>>> Is this really a good idea? Why have a control flow graph if it doesn't actually capture control flow? There are lots of compilers for languages with more pervasive exceptions that represent them explicitly, e.g. the Hotspot server compiler for Java or several ML compilers (where integer overflow throws an exception).
>> 
>> You and Bill seem to be responding to a different question, namely "Is LLVM expressive enough to represent synchronous exceptions from non-call instructions?"  This really has nothing to do with Sohail's question.  Duncan is quite correct:  the only reasonable representation for asynchronous exceptions is to attach EH information to basic blocks.
>> 
> Placing the EH information on the basic block has the same implications for the CFG for both questions.

Let me make an analogy.  We live in Germany.  Sohail wants to drive to Spain.  Duncan told him to go through France.  You and Cameron are saying that the traffic in France is awful, and some friends who went to Italy didn't go through France.  I am trying to point out that Italy is not Spain, even though they are both on the Mediterranean, and that you have to drive through France to get to Spain.

There is really no alternative to putting EH edges on basic blocks if you're going to support preemptive asynchronous exceptions — some random multiply that gets hoisted out of a loop has to change exception handlers just in case that's where the PC lands during a signal.  There isn't much point in complaining that doing so muddies the CFG, which is really just an inherent fact of handling asynchronous exceptions.  That is not true for synchronous exceptions;  you don't have to abandon the "internally throwing instructions are terminators" design at all, you just have to allow more things to be terminators.

John.